Ireland's justice minister warned today that the threat from dissident republicans is severe after police foiled a major terrorist attack at the weekend.

Gardaí raided a Real IRA bomb factory near Dundalk, County Louth in the Irish Republic on Saturday evening. They found two gas cylinders mounted on a small trailer which they believed were to be transported across the border a few miles away and used in a bomb attack in Northern Ireland.

Two men, one in his 20s and the other in his mid-50s, are being questioned by the Garda Síochána's special detective unit.

Detectives investigating Real IRA activity on the Irish border believe the gas cylinders had been modified to store explosives. The raided property is close to the M1 motorway linking Belfast and Dublin.

Dermot Ahern, the Irish justice minister, said the find had uncovered "a fairly major engineering operation in progress".

He sad: "It was an operation designed to let off a bomb somewhere and obviously it would be anticipated that the bomb would have been transported across the border. It again exemplifies that this [threat] is severe."

His counterpart in Northern Ireland, the alliance minister David Ford, praised the gardaí for finding the bomb parts.

"There clearly has been excellent co-operation between gardaí and the PSNI [Police Service of Northern Ireland] in recent months and I welcome this further evidence of this continuing work," he said.

In other developments in Northern Ireland a second cousin of the Sinn Féin president Gerry Adams starts his 40th day on hunger strike today after signing legal papers stating that he should not be revived if he slips into unconsciousness.

Republican dissident prisoner Liam Hannaway started his fast inside Maghaberry top security prison outside Belfast as a protest against being held on the prison's special segregation unit. He wants to be moved into the republican wings.

Hannaway's grandfather Liam was one of the founders of the Provisional IRA.

Supporters of the prisoner, who belongs to the republican splinter group Saor Uladh, which is opposed to the peace process, confirmed that he has left instructions that he should not be revived in the prison hospital.

Carl Reilly of the Republican Unity Network said he had spoken to Hannaway's father on Friday evening. Asked about reports from the jail over Hannaway leaving instructions about what to do if he becomes unconscious in the prison hospital, Reilly said: "Yes, I am led to believe that."

Reilly said that the republican also suffers from a congenital heart condition which runs in the Hannaway family.

"We are entering a critical phase and Liam Hannaway was already being treated for coronary problems even before he went on his hunger strike. The prison authorities and the Northern Ireland Office have been trying to play this situation down but now it has reached a serious turning point," he said.

A spokesperson for the Northern Ireland Prison Service claimed this weekend that Liam Hannaway had not applied for a transfer within the jail even though both supporters and family claim this is his key demand.

A prison service spokesman said: "Mr Hannaway, who is being housed in the vulnerable prisoners unit, appears to have some personal issues surrounding prisoners' food and visiting arrangements. He is also believed to be unhappy at his location within the prison.

"The prison service is attempting to work through the issues of concern with Mr Hannaway. However, the safety of prisoners is paramount and the prison service has received intelligence of a specific threat against Mr Hannaway. We cannot comment on the nature of the threat.

"Mr Hannaway has not applied to be placed in separated conditions."

The last deaths from a hunger strike in Northern Ireland were in 1981 when seven IRA prisoners and three INLA inmates died.